The thorium fuel cycle is based on the use of the common isotope Th-232 as what is called the fertile material in the reactor. Natural thorium is not fissile meaning that it cannot create self-sustaining reactor. The Th-232 must be transmuted into the fissile material, U-233, via neutron capture. The U-233 is the actual nuclear fuel for the reactor.
When bombarded by slow neutrons, Th-232 atoms absorb a neutron and becomes Th-233. The Th-233 atom then emits an electron to become protactinium-233. The Pa-233 atom then emits an electron and becomes U-233.
Ninety two percent of the time, when a thermal neutron hits a U-233 atom, that atom will split into lighter fission produced atoms. The other eight percent of the time, the atom absorbs the neutron and becomes U-234. At each stage of neutron collision, a small percent of collisions produce transmutation so diminishing at each stage from U-234 to U-235 to U-236 to Np-237 to Pu-238 to Pu239 and heavier isotopes of plutonium. The plutonium can be used as a fuel and the other elements produced can be extracted as waste or recycled into reactors be converted into lighter fission products.
Protactinium-131 can interact with Th-232 yielding Th-231 which decays to Pa-231. Pa-231 is radioactive and has a half-life of 3,270 years. It is a major contributor to long term radioactive waste. This production of radioactive wastes from thorium reactors is much less than the production of radioactive wastes in reactors fueled by uranium or combinations of uranium and plutonium.
Uranium-232 is also produced in the thorium fuel cycle. It has a half-life of about seventy years and its decay path through Th-228 produce high energy gamma radiation .The hard gamma radiation produced by the thorium fuel cycle can harm electronics and requires remote handling during reprocessing Chemical removal of the Th-228 from the uranium can remove the gamma radiation threat.
Thorium is three times as efficient at absorbing thermal neutrons as U-238. The U-233 captures fewer neutrons than U-235 and Pu-239 meaning that thorium makes more economic use of neutrons. And, the U-233 emits more neutrons than it absorbs so thorium reactors can be used to breed more uranium fuel.
Thorium oxide used in thorium reactors has a higher melting point, higher thermal conductivity, less thermal expansion and greater chemical stability than the uranium oxide used in conventional uranium reactors. The U-233 that a thorium reactor produces is contaminated with U-232 which makes it useless as a source of feed stock for nuclear weapons materials. Thorium reactors produces much less of the heavily radioactive transuranics produced by uranium reactors.
Thorium fuel is harder to fabricate than uranium fuel. The Pa-233 that develops in thorium fuels reduces the neutron economy. Once-through thorium reactors produces long lived radioactive waste. Recycling thorium fuel requires special handling to deal with the high levels of gamma radiation.
